More than 40 different types of cells have been identified in the lung, reflecting a diversity of metabolic activity. Advances in our understanding of the biochemical functions of the lung will depend on progress in in vitro culture studies. Appropriate conditions for culturing pulmonary epithelial cells are lacking. Therefore, the initial goal of the Pulmonary Cell Biology Group is to develop culture conditions for these epithelial cells. We have observed in a primary lung cell culture that the maintenance of epithelial cells in culture depends on medium conditioned by the mouse BalbC 3T3 cells. With this conditioned medium, the growth of fibroblastic cells is retarded, while epithelial cells grow slowly but steadily. More than 30 epithelial clones were selected. In the preliminary study, two clones, CO5 and DO7 differing in their clonal morphologies were recloned and further studied. Results have shown that both cells depend on the conditioned medium for plating at low cell density. Furthermore, CO5 shows a dependence on the conditioned medium for growth. These results suggest that the conditioned medium can be used for the clonal isolation of different types of pulmonary epithelial cells and for the maintenance of these cells in culture.